A key unit is a kind of parts composing a mobile equipment such as a mobile phone, a portable digital assistant device (PDA) and so forth wherein many switch operation keys (push buttons) are assembled and arranged on a piece of sheet. A key is composed of a key top made of hard resin or the like, and a keypad made of soft rubber such as silicone rubber, or thermoplastic-elastomer. The key top is installed on the surface of the keypad, and a switch pressing projection (what is called a “plunger”) is formed on the plane (reverse side) opposite to the plane of the side of the keypad where the key top is installed. A space between each key is connected by the keypad. If a printed circuit board having switch elements is placed on the undersurface of the key unit being composed this way, a key switch may be formed in the position corresponding to each key.
Characters or symbols indicating the function of each key are drawn on the key top. Such process for drawing characters or symbols on the key is called as “marking”. Since each of these keys is arranged in the most noticeable place in the target mobile equipment, the particular attention is paid to the design and arrangement thereof. Particularly, durability and high-class impression are important elements. Although a transparent glass key top has never been utilized for a mobile equipment such as a mobile phone and so forth in the past, it seems that the transparent glass key will possibly become an important candidate to be utilized for a mobile equipment hereafter from the above-mentioned viewpoint.
However, a transparent glass key top (hereinafter referred to as a “glass key”) has a problem to be solved. The problem relates to a difficulty in marking on the glass key. Of course, it can be considered to form a character or symbol on a surface of a glass key by printing or painting, however, it is not easy to assign wear resistance to the character or symbol printed on a hard and smooth glass surface.
There is an example of having tried to solve the above-mentioned problem (e.g., refer to the patent document 1). According to this example, a character and so forth are engraved by removing the fine cracks formed on the glass surface due to the sudden change of the temperature at the irradiated spot resulting from irradiation of carbon dioxide laser to a glass surface.
Patent document 1:
Unexamined patent publication No. Hei 10-291840 The object of the patent document 1 is not the same as a glass key as claimed in the invention.